Cycling builds the bacteria your tank needs to keep fish alive

Cycling is the process of growing beneficial bacteria in your aquarium water and filter so they can break down fish waste. Without these bacteria, ammonia from fish waste builds up to toxic levels and kills your fish within days. Cycling takes two to four weeks and happens before you add any fish. You run the tank empty (or with plants and decorations only), add an ammonia source, and test the water every few days until the bacteria colonies are established and working.

The bacteria you need are Nitrosomonas (which converts ammonia to nitrite) and Nitrobacter (which converts nitrite to nitrate). Nitrate is far less toxic and is removed during water changes. Once these bacteria are thriving, your tank can handle the ammonia fish produce every day.

Key Takeaways

  • Cycling takes two to four weeks and must happen before you add fish, because ammonia from fish waste is toxic without bacteria to break it down.
  • You add an ammonia source (fish food, pure ammonia, or ammonia chloride) to feed the bacteria while the tank cycles.
  • Test ammonia, nitrite, and nitrate every two to three days using a liquid test kit, because color-change strips are too unreliable for cycling.
  • Cycling is complete when ammonia and nitrite both drop to zero within 24 hours after you add ammonia, meaning the bacteria are working fast enough.
  • You can speed up cycling by adding filter media or substrate from an established tank, but the process still takes at least one to two weeks.

Set up your tank and filter before you start

Install your filter, heater, and air stone or powerhead before you begin cycling. The filter is where most of your beneficial bacteria will live, so it needs to be running the whole time. Set your heater to the temperature range for the fish you plan to keep—most tropical fish need 76 to 78 degrees Fahrenheit. Bacteria grow faster in warmer water, so a heated tank cycles faster than an unheated one.

Fill the tank with dechlorinated water. If your tap water contains chlorine or chloramine, use a dechlorinator product (follow the bottle instructions) or let the water sit uncovered for 24 hours if it contains chlorine only. Chloramine does not evaporate and must be treated. Run your filter and heater for a few hours to make sure everything works before you add ammonia.

Add an ammonia source to feed the bacteria

You need to add ammonia to the water so the bacteria have something to eat while they grow. You have three main options: fish food, pure ammonia, or ammonia chloride.

Fish food is the easiest and safest method. Sprinkle a small pinch of flake food into the tank once a day. The food decays and produces ammonia naturally. This method is slower than the others but works reliably and is hard to overdose. Add just enough that it mostly dissolves by the next day—if food is piling up on the bottom, you added too much.

Pure ammonia (the clear liquid sold for cleaning, not the scented kind) works faster. Add a few drops at a time until the ammonia reading on your test kit reaches 2 to 4 parts per million (ppm). This method is quicker but requires a liquid test kit to avoid overdosing. Do not use ammonia with added dyes or perfumes.

Ammonia chloride is a powder sold specifically for aquarium cycling. It dissolves completely and does not leave residue. Follow the package instructions to reach 2 to 4 ppm ammonia. This method is precise and fast but requires ordering online or finding a specialty aquarium store.

Test water every two to three days with a liquid test kit

Buy a liquid test kit that measures ammonia, nitrite, and nitrate. Brands like API Master Test Kit are widely available at aquarium stores and online. Do not use color-change strips—they are too inaccurate for cycling and will give you false readings that waste weeks of time.

Test the water every two to three days and write down the numbers. You are watching for three things: ammonia should drop, nitrite should rise and then drop, and nitrate should rise. In the first week, ammonia may stay high or drop slowly. In the second week, you should see nitrite appear and ammonia start to drop. By week three or four, nitrite should drop to zero and nitrate should be present.

If ammonia stays above 4 ppm after the first week, do a 25 percent water change to bring it down. High ammonia slows bacteria growth. If ammonia is still at zero or near zero after a few days, add more ammonia or a pinch of food—the bacteria need food to survive.

Recognize when cycling is complete

Cycling is finished when ammonia and nitrite both reach zero within 24 hours after you add ammonia. This means the bacteria are working fast enough to process all the ammonia you add before the next test. Nitrate will be present (usually 5 to 20 ppm or higher), which is normal and expected.

Do a 50 percent water change to remove excess nitrate before you add fish. This gives you a clean starting point. After the water change, test again to confirm ammonia and nitrite are still zero. If they are, your tank is ready for fish.

Speed up cycling with bacteria from an established tank

If you have access to filter media, substrate, or decorations from an aquarium that has been running for at least a month, you can cut cycling time in half. Beneficial bacteria live on these surfaces, so moving them to your new tank introduces the bacteria directly.

Rinse the used filter media in old tank water (not tap water, which kills bacteria) and place it in your new filter. Or add a handful of substrate from the established tank to your new substrate. You still need to add ammonia and test the water, but the bacteria are already present and will multiply faster. Cycling usually takes one to two weeks instead of three to four.

If you do not have access to an established tank, some aquarium stores sell bottled bacteria cultures. These products vary in quality—some work well and some do not. A liquid test kit is your only way to know if the product is helping, so do not rely on it alone. Continue testing every two to three days as if you were cycling normally.

Avoid common cycling mistakes

Do not add fish before ammonia and nitrite both reach zero. Even hardy fish like goldfish or bettas will die in a cycling tank. Ammonia and nitrite are toxic at levels as low as 1 ppm over time. Wait for the bacteria to do their job.

Do not do large water changes during cycling unless ammonia climbs above 4 ppm. Water changes remove ammonia, which slows bacteria growth. Small changes (10 to 25 percent) are fine if ammonia is too high, but frequent large changes will restart the cycle.

Do not turn off the filter or heater. Bacteria need oxygen and warmth to grow. If the power goes out, the cycle may restart or slow down significantly. Keep the filter running 24 hours a day.

Do not add decorations, plants, or substrate after you start cycling. These can introduce new bacteria or organic matter that changes ammonia levels and makes testing unreliable. Add everything before you add ammonia.

Frequently Asked Questions

Can I cycle a tank with live plants instead of ammonia?

Live plants consume ammonia directly, so they can reduce the ammonia available for bacteria to eat. A heavily planted tank may cycle slower or not at all because the plants are doing the ammonia removal instead of the bacteria. If you want plants, add them after cycling is complete, or use a small amount of ammonia alongside the plants and test regularly to make sure bacteria are still growing.

What if my ammonia and nitrite are both zero but nitrate is also zero?

This usually means the bacteria have not grown yet, or you did not add enough ammonia for them to eat. Add more ammonia or food and test again in two to three days. If this pattern continues for more than a week, your water may be too cold—check that your heater is working and set to at least 76 degrees Fahrenheit.

Can I add fish slowly while the tank is cycling?

No. Even one or two fish produce enough ammonia to spike the tank if bacteria are not fully established. Wait until ammonia and nitrite both drop to zero within 24 hours, then add fish. You can add a few fish at a time after that, but the cycle must be complete first.

How long does cycling take if I use bottled bacteria?

Bottled bacteria products vary widely in effectiveness. Some reduce cycling time to one to two weeks, while others do not help at all. The only way to know is to test the water every two to three days. Do not assume the product is working—use your test kit to confirm ammonia and nitrite are dropping on schedule.

Do I need to do water changes during cycling?

Only if ammonia climbs above 4 ppm. In that case, do a 25 percent water change to bring it down. Otherwise, avoid water changes because they remove ammonia that the bacteria need to grow. Once cycling is complete, do a 50 percent water change before adding fish to remove excess nitrate.